View Full Version : Power supply voltage
03-26-1999, 10:00 PM
I have seen several opinions and suggestion regarding power supply upgrades. Now that those who have done these upgrades have had a chance to use their machines, I would like to know who is having positive results and what modifications were made. What are the real world observable advances that these modifications achieved?
04-18-1999, 10:42 PM
I have a shopbot equiped with the larger motors, dual drivers, and an external 12v power supply from shopbot. I have converted to a rack and pinion system of my own with a 25% higher gear ratio than supplied by Shopbot. With this setup I was getting a lot of vibration at move speed ( 1 inch/ sec)and could not jog over 3.5 inches/sec without motor stall.
Today I installed a surplus 24v 25amp power supply. I had incredible results. The vibration at move speed decreased dramatically. The motor did'nt stall at any jog speed. I even set the ramp to .1 and jog to 7 to try to find it's limits, it jerked a bit and then came right up to speed.
Overall the machine runs much smoother and quieter over the entire speed range.
05-11-1999, 03:31 PM
I tried a 24v 7 amps supply and toasted my Y channel driver board, the x board also started to burn but is not dead (yet?). Does anybody know why this might have happened? The supply i used is a simple toroidal (doughnut) transformer with a 7 amps rectifier, the current at the points reads a stable 24 v. Should i have used anything more complex than that for a PS? Could it be that the channel boards i have are not to handle that voltage? I bought my machine around november 1997. Is there possibly any other sort of electrical "interference" that could burn a board? Also i checked and there is no short at the motors. Does anybody have any ideas?
05-11-1999, 04:25 PM
When a 12 or 24 volt power supply is used, the drivers and circuit board should still be powered by the original built-in power supply that comes with the ShopBot box; the higher voltage should be supplied to the motors only ( via the black and yellow wires I believe).
There's a good discussion about power supplies under the heading VARIATIONS AND MODIFICATIONS OF SHOPBOTS; Info on Power Supplies.
05-11-1999, 08:45 PM
I did that connection, black/negative and yellow/positive separately from the logical circuit. I could not believe when i saw the smoke coming out of the boards, mostly because i had already tried another 24 v supply (a much weaker one)and things worked fine. That supply burned on its own, because i was recomended making a bridge between two sections of it to get 24 v, but it had a logical circuit that did not handle the load and went up in smoke. But even after that accident the circuits worked fine with the original supply back in line.
My problem was getting a supply down here in Brazil, so i got parts and made "my own", and as i said its output tested fine prior and after burning the boards. I do not think my Y driver board can be fixed, and my X board is sort of burned, i do not think it will handle 24 V ever, i sort of lost confidence since i don't know much of electronics, and can't figure out what's happened.
One of the things i think is if there is anything like a sort of weird frequecy ressonance (?) since my supply now is really simple consisting of a doughnut shaped transformer and a siemens 7 amps rectifier. I was told that toroidal trasformers are quite "clean", and the one i got seems to have plenty of amps, being limited only by the rectifyer. I figured it would be better to burn the rectifyer than my control box, since i know that the recomended amperage is around 18 amps. I was going to rig 3 such PSs one for each channel, after this unlucky test.
I am very much in the dark here, and i thank you Bill and anyone that may have an input on what might have happened, because i want to go 24 v but now i am totally afraid. I dont like the smell of burning boards in the morning...
A rectifier and a transformer will give you DC voltage, but it is a pulsating D.C. and not a pure D.C. volatge, it has a 120 cycle ripple (twice the line frequency, if you have 50 cycle power then 100 cycle ripple) to it if you are using a full wave bridge rectifier.
Are you using a full wave bridge or a half wave or just a single diode? What is the part number of your seimens bridge? Are you sure you had the bridge wired up properly? Did you have the right polarity to the motor circuit?
Did you measure the output of your power supply before hooking it up to the shopbot motor circuit?
You must also hook up a large electrolytic capacitor to smooth out the volatge of the rectifier.
Ted Hall, ShopBot
05-13-1999, 02:37 PM
I think Phil is on the right track here with the concern about getting a true/smooth DC voltage. It's hard to diagnose the circuit at a distance. You need to be checking with an oscilloscope in this case, since we aren't sure of the nature of the supply. Using a standard multimeter will give you averaged and smoothed voltage values and little indication of the actual nature of the supply provided.
Your problem may also be related to the circuit's grounding. Your ground may be floating and/or not referenced to the same 0 as the 5v logic supply. Indeed, for the boards to smoke, I think that you must probably have some sort of ground short.
Also, your power supply, as Phil indicated, should have some sort of large capacitor for smoothing. In addition, because of the pulsating nature of the PWM load of the driver boards, you should put something like a 440uf/electrolytic capacitor across the inputs right at the first driver board (or better, 1, 100uf on each driver; be careful to get the polarities right).
There may be hope for the smoking board ... turn it over and have a look at the traces. If it's a grounding short, there is probably a trace that acted as a 'fuse' and created the smoke. You may have some luck soldering a jumper across the part of the trace that is melted away. Probably only a 30% shot, but worth a try. (Don't ask me why I know this one.)
Let us know what you need and we can get new driver boards out to you.
05-13-1999, 02:54 PM
Phil, thanks for the interest,
The rectifier i got is not Siemens it is a Brazilian brand, Semikron (small prints...) But it is a full wave - rectifier (bridge?) It is a small square thing measuring on two sides about 1 1/4" by 3/8 high, with both negative and positive AC inputs (~) and related DC outputs. I did measure the output, and it was and still is a steady 24 v. The assasin PS is well and dandy. I did not try the electrolytic capacitor, i was told that toroidal transformers are quite "smooth". I shall try the capacitor. I wonder if because i did try this PS on all tree channel circuits the demand load might have been much more than what the rectifier could handle and that overload instead of burning the bridge made the DC into AC , if i can put it this way.
I was pretty sure that i connected the polarity right, i checked it more than once before turning things on but to this point i am willing to think i did not. The thing is that what burned on the boards apparently was not one of the components but the circuit itself. Would the polarity reversal do that? After all the boards are rated 94 v on their backs.
I will be buying me a new channel board for Y, but before i understand what i did wrong, if that was the case, i dont think i'll be going over 12V.
05-13-1999, 03:07 PM
I will be trying the capacitor, and also i will be checking for that short. I already did try that bridging on the circuit traces, no such luck here. I am worried because the board for X suferred a bit also but it seems to be working.
I think the ground short is a good explanation, because right before the smoke there was a spark on the board. How can i make sure i have everithing properly grounded? What is a floating/referenced ground?
I will be contacting you by e-mail to get a new driver board.
05-13-1999, 05:11 PM
I forgot to tell you the bridge spec # it is SKB7/02.
Ted Hall, ShopBot
05-18-1999, 12:15 AM
As a starting point (w/o the oscilloscope) compare the negative outputs of the two power supplies by measuring the voltage across them ... this may reveal the problem.
Should you want to see how the 24v works on the tool, you can always just put two 12v car batteries in series ... not good for regular use or for the batteries, but it may give you an idea of the improvement the 24v will give you and indicate whether its worth your effort.
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